Background: While epidemiological studies have demonstrated an association between ambient particulate pollution and adverse health effect, uncertainties still exist concerning specific properties of particles that affect pathophysiological mechanisms. Often it has been very difficult to reproduce ambient conditions in a laboratory with sufficient doses to elicit effects. The development of ambient particulate concentrators has bridged this gap, effectively merging the ambient environment with the laboratory. Concentrated ambient particle experiments have linked real-world particulate matter to a host of health effects observed in human and animal exposure studies as well as in vitro. Specific Aims: The objective of this 5-year project core, under the leadership of Dr Contantinos Sioutas, is to support project investigators and Center members who will be investigating the effects of ambient particulate matter on human health, including allergic inflammation. This core will provide chemically-characterized, concentrated ambient fine and ultrafine particle suspensions collected in downtown Los Angeles. The proposed project expands upon the NIEHS and US EPA funded research concerning the adverse health effects of particulate matter that has taken place at USC and UCLA over the past five years. Methods: Concentrated ambient fine and ultrafine particles will be collected by means of new and improved portable concentrators developed over the past five years by the University of Southern California, at a typical urban background site in downtown Los Angeles. The majority of the concentrated flow will be collected in aqueous suspension, immediately frozen with dry ice and transferred to UCLA for toxicity analysis. This procedure minimizes any possible chemical reactions in the suspension, maintaining its realworld characteristics. Aerosol characterization will include mass and chemistry measurements, including trace elements, inorganic ions, elemental/organic carbon, and PAHs. Relevance: The use of aerosol concentrators in conjunction with various in vitro and in vivo toxicity assays over the past five years has significantly improved researchers'understanding of health effects due to particles. Now that certain health endpoints have become the focus of experiments, the mechanisms leading to these endpoints can be investigated. Past studies have employed concentrators at various locations and seasons while emphasizing a variety of endpoints. By controlling these variables and enlarging the sample set, exploration into specific biological mechanisms behind observed health effects of particulate matter can begin.